8.3 - Qualitative Analysis

Question 1

What are the 3 anion tests?

Answer 1

Carbonate
Sulfate
Halide

Question 2

Describe the carbonate test.

Answer 2

Carbonate + acid > CO2

1. Add dilute HNO3 to solid/solution
2. Bubbles form > could be a carbonate
3. To prove the bubbles are CO2, bubble the gas through limewater, if it goes cloudy it is CO2

CO2(g) + Ca(OH)2(aq) > CaCO3(s) + H2O(l)
Calcium carbonate precipitate shows presence of CO2

Question 3

Describe the sulfate tests.

Answer 3

Sulfates are soluble in water, BaSO4 is insoluble white precipitate
1. Add Ba2+(aq) to unknown compound
2. White precipitate forms
Ba(2+)(aq) + SO4(2-)(aq) > BaSO4(s)

Question 4

Describe the halide test.

Answer 4

1. Ass AgNO3(aq) to X(aq)
2. AgCl(s) - white, AgBr(s) - cream, AgI(s) - yellow

AgCl - soluble in dilute NH3(aq)

AgBr - soluble in conc. NH3(aq)

AgI - INSOLUBLE in conc. NH3(aq0

Question 5

What is the sequence of anion tests and why?

Answer 5

Carbonate, Sulfate, Halide

1. Eliminate possiblity of CO3(2-), neither sulfate, nor halide with HNO3 acid gives bubbles
2. Why eliminate? If there are carbonate ions, a white BaCO3 forms, how do you know which is which.
3. With halides, you are looking for a precipitate, Ag2CO3 and Ag2SO4 are both insoluble precipitates (hence halide test is last)

Question 6

Describe the process of testing a mixture of ions.

Answer 6

Same sequence: Carbonate, Sulfate, Halide
1. CO3(2-) - keep adding HNO3 until bubbling stops - ensures no more carbonate ions (otherwise precip. forms in subsequent tests)
Use HNO3, not H2SO4 or HCl, they will show up in sulfate and halide tests

2. SO4(2-) test - add excess Ba(NO3)2(aq)
   - Filter precip of BaSo4(s), if you want to test for halide ions, don’t use BaCl
3. Proceed with halide test as normal

Question 7

Describe the test for NH4(+) (cations)

Answer 7

1. NH4 + NaOH(aq)
2. NH3(g) is produced - no gas bubbles as NH3 gas is soluble in water
3. Warm the mixtiure, NH3(g) is released
4. You can smell the NH3(g) or moist pH indicator paper turns red to blue.